What is a composite laminate layup and why does ply orientation matter?

A laminate layup is the stacking sequence of composite plies specifying fiber orientation, typically noted as [0/45/-45/90]s (s indicates symmetric). Ply orientation matters because composites are anisotropic - properties depend on direction. 0-degree plies carry longitudinal loads, 90-degree plies carry transverse loads, and +/-45-degree plies carry shear. Quasi-isotropic layups (equal proportions of 0, +/-45, 90) approximate isotropic behavior. Layup design matches fiber directions to principal load paths for efficiency. Symmetric layups prevent warping during curing; balanced layups (+/- pairs) prevent shear coupling.